1. Field of the Invention
This invention relates to a semiconductor exposure apparatus used in an integrated circuit manufacturing process and having chucks designed to suppress the magnification error for the pattern of the preceding step when the pattern of a mask is transferred onto a semiconductor wafer.
2. Description of the Prior Art
In integrated circuit manufacturing, the step of projecting and transferring a mask pattern onto a semiconductor wafer substrate is done plural times. At this time, it is necessary that the mask pattern of the next step be highly accurately aligned over the entire surface of the wafer relative to the mask pattern transferred onto the wafer in the preceding step.
Generally, a semiconductor wafer is adsorbed to a wafer chuck which is a support means for fixedly supporting the wafer and, once the wafer is held to the wafer chuck, the vacuum is not released until exposure is completed. Heretofore, the wafer chuck has been made of a metal, such as aluminum or stainless steel, which is very sensitive to the thermal expansion and contraction caused by fluctuation of the ambient temperatures as compared with the wafer. That is, it has been confirmed that when the wafer chuck creates a minute amount of expansion and contraction with the wafer held to the wafer chuck, the wafer also creates expansion and contraction exceeding the elongation percentage intrinsic to the wafer. For example, let it be assumed that the temperature around the chuck has risen by 0.2.degree. C. from after a silicon wafer has been adsorbed to a wafer chuck made of aluminum until exposure is started, and the temperatures of the wafer and wafer chuck have also risen correspondingly. Heretofore, in such a case, due to the coefficient of linear expansion (2.5.times.10.sup.-6 /.degree.C.) of the silicon wafer, the amount of expansion of a wafer of 5 inches has been only about 0.06 .mu.m and this has been considered to be a negligible amount. At this time, the wafer chuck (whose coefficient of linear expansion is 23.times.10.sup.-6 /.degree.C.) expands to about 0.6 .mu.m and the wafer is pulled by the friction force with the chuck and exhibits an amount of elongation approximate thereto. Accordingly, if exposure is effected in such condition, a magnification error occurs between the pattern of the preceding step and the pattern of the current step and highly accurate alignment of the wafer over the entire surface thereof becomes impossible. In recent years, various methods of highly accurately aligning a mask pattern and a wafer pattern by controlling the temperature of the wafer have been devised. However, if the temperature of the wafer chuck is changed after the wafer has been fixed to the wafer chuck, a great amount of elongation occurs in the wafer chuck which therefore, the wafer is pulled thereby, and has led to the disadvantage that an amount of expansion and contraction greater than necessary occurs.
For this reason, it has been very difficult to achieve highly accurate alignment of the wafer over the entire surface thereof in a case where use is made of a wafer chuck made of a metal such as stainless steel or aluminum.
A similar tendency seems to occur not only in the wafer and wafer chuck but also in the mask and mask chuck although there is difference in degree.